You don't want to go medicating them if you don't know what the problem is. It can't be the weather since it's been so mild lately. Any vet in theory should be able to do a fecal float test to see if there any parasite eggs. Just tell them it is from your cat and hand them a teaspoon of fresh chicken poo mixed up from a few fresh piles. J/k.

Dunno what else to say really other than hopefully it's just the moon or something.